This invention relates to the orientation of monocrystalline materials, and more particularly, to the sawing of such materials with predetermined crystallographic orientation.
One step which is typical in the processing of semiconductor materials, such as single crystal silicon pulled by the Czochralski process from a melt of polycrystalline silicon, involves mounting a generally cylindrical single crystal (monocrystalline) rod or so-called "billet" and then sawing the billet into thin wafers or slices. This is referred to in the art as "slicing." It is desirable to cut in a precisely predetermined direction with respect to the various crystallographic planes of the material. A specific explanation of the reasons and other considerations which are involved in crystallographic orientation of silicon are described in the article by D. O. Townley, "Optimum Crystallographic Orientation for Silicon Device Fabrication," Solid State Technology, Jan. 1973. It may be preferred to saw the material with the saw blade either parallel to a specific crystallographic plane or tilt it at a small angle, e.g., 4.degree.-5.degree., from a specific plane.
Crystallographic orientation for this purpose can be ascertained precisely using x-ray diffraction methods. X-ray equipment is cumbersome and expensive and, in addition, requires time-consuming transfer of a semiconductor rod or billet to be sliced from the x-ray equipment to the saw. Thus, as a practical matter, it is inconvenient and costly to use x-ray diffraction orientation. OpticaL orientation can instead be employed. For this purpose, a hot caustic or acetic acid etch is used to develop small facets in a face of the monocrystalline material. These facets preferentially expose certain crystallographic planes. The etched surface then reflects light in directions determined by the crystallographic planar orientation, rather than the plane of the original cut surface or face of the material. Methods other than etching, such as grinding, abrading or the like, can be used to preferentially expose specific crystal faces.
In one prior art commercial use of optical orientation for slicing, the slicing process includes the steps of rod mounting (i.e., mounting of the billet), orientation, and sawing. In rod mounting, the rods are glued to blocks of a suitable material which will hold the slices in place after they have been sawed. Mounting is tied to orientation in this process since the rods must be located in specific geometric relationship to the blocks in order that the slices can be sawed in the specific crystallographic orientation. Orientation is achieved by tilting the rod with respect to the blade plane in a fixture which holds the billet or rod by clamping the block. The angles of tilt are first set by aligning the optical reflection pattern of an etched end of the rod. Final adjustment is made after the slice has been cut and its exact angle to the crystallographic axis determined by x-ray diffraction.
As will be known to those skilled in the art, sawing of monocrystlline materials such as silicon is quite often carried out using an inside diameter (I.D.) saw. An I.D. blade is typically a very thin, annular stainless steel disk having cutting materials such as diamond dust in a matrix coating the inside cutting edge of the blade.
A device useful in carrying out the optical orientation in this commercial process is commercially available from the Sylvania Company and is known as the Sylvania Crystal Orientation Instrument. Its use typically requires objectionable transfer of the rod to be sawed on a fixture from the instrument to the saw.
Other instruments are all also commercially available which can be used for optical orientation for crystals in sawing. One of these devices, which is commercially available from the Carl Zeiss Company, Oberkochen, Wuerttemberg, Germany, is basically an autocollimating telescope. However, none of these are fully satisfactory when used in conjunction with a saw, either because they lack portability, are not self-contained, provide poor optical performance, or their accuracy is dependent upon precise positioning or distance of the device from the monocrystalline material which is to be optically oriented, or rerequire transfer of the rod to be sawed between the instrument and the saw. These prior art devices are typically not truly compact, light in weight, simple, and fully reliable when used to align crystals for sawing.
However, it is believed that heretofore there has not been known or suggested in the art a small, portable device which can be attached to body of monocrystalline material to be sawed and which device includes dual autocollimators having parallel collimation axes with one autocollimator directing a collimated beam of light on the body of material and the other directing a collimated beam of light on the saw blade.